1. Field of the Invention
The present invention relates to porous multilayered hollow fibers and a filtration module having the porous multilayered hollow fibers, and a method of manufacturing the porous multilayered hollow fibers. Particularly, the invention relates to the improvement of filtration properties of porous multilayered hollow fibers which are composed of porous tubes such as the polytetrafluoroethylene and which are used in filtration equipment for solid-liquid separation treatment in the fields of environmental preservation, pharmacy, and foods.
2. Background Art
Porous materials consisting of polytetrafluoroethylene have characteristics such as nonstickiness and low coefficient of friction in addition to excellent chemical resistance, heat resistance, weather resistance, fire resistance, etc. Also, because of their porous structure, they exhibit not only excellent permeability and flexibility but also superior performance in catching and filtration of fine particles. In the past, therefore, the materials consisting of polytetrafluoroethylene have been used extensively in the fields such as filtration of precision chemical medicines, and filters for drainage processing.
More specifically, the porous polytetrafluoroethylene materials are shaped into a tubular or sheet form and used for various filters, deaerator membranes, waterproof membranes, etc., since they have microscopic fibrous texture in which fibrils are interconnected in the form of a three-dimensional network such that numerous pores exist surrounded with fibrous frameworks, which are constituted of nodes and fibrils. Also, it is possible to make a filtration module for solid-liquid separation treatment by bunching plurality of tubes made of such porous polytetrafluoroethylene into a modularized form.
Various proposals have been made for achieving high filtration property of porous polytetrafluoroethylene filtration materials to be used in such filtration module.
For example, Japanese Patent Application Publication H4-75044B discloses a tubular filter in which an attempt is made to achieve filtration property with a continuous porous film made of polytetrafluoroethylene and to improve the tube strength by wind-covering the film on the outer surface of a continuous porous tube made of polytetrafluoroethylene. In addition, a tubulous filter in which a porous film has filtration property to remove minute particles of 0.1 μm or more is proposed.
Japanese Utility Model Application Publication H4-3607 proposes a tubular filter which filters fluid from its inner side to outer side and which is constituted of a porous polytetrafluoroethylene tube, a porous polytetrafluoroethylene film whose pore size is smaller than that of the tube, and a reinforcement knit layer of polytetrafluoroethylene yarn. The film is spirally wound, extending in the axial direction, over the outer surface of the tube, and the reinforcement knit layer is provided on the outer surface of a layer made of such wound film.
Moreover, in Japanese Patent No. 3221095 the present applicant has proposed a porous multilayered hollow fiber in which a fluid-permiable sheet such as a porous polytetrafluoroethylene sheet is fixed on the inner surface of a porous polytetrafluoroethylene tube by hot-melt adhesion.
A drawback of the tubulous filter disclosed in Japanese Patent Application Publication H4-75044B is that the filtration performance deteriorates with the elapse of time because pores of the porous film are gradually filled with particles as a result of hours of usage, although the filter has a function of removing minute particles of 0.1 μm or more.
That is, the filter is structured such that minute particles are not removed at the surface of the porous film, but 0.1 μm minute particles are removed by the porous film as a whole, since the pores of the film capture the minute particles when they pass inside the pores, which are designed to be relatively large enough to attain a flow rate of some degree. Therefore, although it is excellent in terms of the filtration property in the initial stage of the usage, its shortcoming is that such good filtration property cannot be maintained for a long time because the clogging occurs with the elapse of time, resulting in the degradation of the filtration property. Particularly, it is disadvantageous in the case of filtering a high-turbidity solution, because rapid clogging occurs.
Generally, the performance of porous materials is determined by filtration precision in terms of their efficiency of collecting the prescribed particles, because it is difficult to define the size of the pores, which are formed with entangled fibrils and are varied in shape and size.
According to Japanese Utility Model Application Publication H4-3607 B filtration is done outwardly from the inner surface of a tube having relatively greater pore sizes such that pre-filtration is accomplished therewith and the precision of filtration is secured with the wound film layer. In the case of filtering a liquid of high turbidity, however, large particles that cannot go out to the outer side clog the pores of the inner layer of the porous tube which performs pre-filtration. Therefore, such clogging deteriorates permeability of liquid. Also, the reinforcement knit layer of yarn provided as the outermost layer does not contribute to the filtration property but deteriorates the flow rate because particles enter into the knit layer.
Thus, the important matter of the membrane filter using a porous material is not only the initial flow rate but also the prevention of degradation in the flow rate due to the clogging caused by solid particles over the elapse of time. That is, what is important is the filtration property in a steady state after some elapse of time in usage.
According to Japanese Patent No. 3221095 the filtration property in a porous multilayered hollow fiber for filtering outward from inner side is improved by unifying the inner side of a tube with a lapped porous sheet having small pore size and high porosity. However, the filtration property is insufficient for filtering liquid of high turbidity because blockage occurs inside a channel. Thus, further improvement is needed.
In filtration using such porous polytetrafluoroethylene tubes, back wash reverse filtration is generally performed to remove particles adhered to the pores or internal surface of the tubes so that the filtration life may be extended. However, in the case where particles are captured not at the surface of the filtration layer but through the whole filtration layer including the thickness of the tubes, particles that have entered into the pores of the porous tubes cannot be removed easily by the back wash reverse filtration. Therefore, the recovery of the filtration property is difficult.
In order to prevent particles from entering into the pores of the porous tubes or the like, it might be considered to decrease the pore size at the surface. In that case, however, the porosity is also reduced inevitably because the expansion ratio is decreased. Thus, the disadvantage is that the permeability for fluid is degraded.
The drawback of such porous polytetrafluoroethylene tubes as described above is that their durability with respect to inner or outer pressure, bending, etc. is insufficient because they are formed such that a polytetrafluoroethylene sheet is simply wound on the plain polytetrafluoroethylene tube. In order to improve the durability, knitting and braiding are made for reinforcement, but such reinforcement is disadvantageous because the flow rate decreases since particles (suspended solid particles) accumulates therein.